Summary

1. Successive rapid responses of the earthworm show a marked tendency to increase in size on repetition.

2. It is shown that this "staircase" phenomenon is not due to peripheral facilitation either on the afferent or efferent side of the reflex, but to summation in the nerve cord and evidence is presented that it occurs at "giant-to-motor" junctions.

3. Facilitation is most pronounced in preparations whose "giant-to-motor" junctions are accommodated. In such cases a single impulse in the median giant fibre is ineffective, two or more being required to produce a rapid response throughout the length of the animal.

4. Fatigue and facilitation in the earthworm is discussed in relation to similar phenomena in other invertebrates.

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